Rechargeable batteries are used extensively to power portable devices such as portable radios, cellular phones and laptop computers.
In certain applications, it may be desirable to use a device powered by a rechargeable battery while the battery is being recharged. For instance, many Land Mobile Radio (LMR) applications have hand-held, portable radios that can be recharged in vehicle-based recharging units. To reduce the number of radios and battery packs, it is highly desirable to use the radios in the vehicle while their batteries are being recharged. It is also desirable that the battery pack remain attached to the radio during recharging, so that when the radio is removed from the charging unit, it is immediately available for portable use.
In a conventional rechargeable battery pack, the recharging line and the device power line are both connected to the same terminal of the cell-stack. When a device attached to such a battery is used during recharging, a single voltage is applied to both the cell-stack and the device. The battery charger can, therefore, only monitor the combined current being supplied to the cell-stack and to the radio. Not being able to monitor the current to the cell-stack alone makes it difficult for the charger to determine the state and rate of recharging. As a result, the battery may be over charged, or charged too quickly. Both of these damage a battery through overheating, and lead to considerably reduced battery life.
This damage may be avoided by deliberately charging the battery at a rate well below the maximum allowable charge rate, and ending the charge well short of maximum battery capacity. This is, however, not acceptable to most industrial and professional users, such as emergency service providers, who are highly desirous of always having the rechargeable batteries in their equipment charged to full capacity.
What is needed is a battery pack having an isolated charging circuit, so that it can be accurately, quickly and fully recharged while attached to a device, and while that device is capable of being operated.